Always use overwrite-only code for non-revert upgrades

[d3zd3z]

We have added the configuration BOOTUTIL_OVERWRITE_ONLY to allow a simpler upgrade process to be used. Instead of just making this code a compile-time choice, enhance the regular startup to use this code when the system has requested a non-test upgrade (which won't be reverted).

Since there is still a desire to be able to revert, this should be another compilation option.

[d3zd3z]

Copied from JIRA MCUB-57.

[mbolivar]

MCUboot is pretty near to its flash partition size (with debug logging enabled) on various targets right now. This might put it over -- just FYI

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